This invention relates to oil recovery using stabilized modified heteropolysaccharides. More particularly, saline heteropolysaccharide solutions containing at least 0.5 wt.% of inorganic salts are stabilized against viscosity loss upon heat treatment if oxygen is removed from the saline solution prior to and during heat treatment. The stabilized solutions are used as mobility control agents for oil recovery.
British Pat. No. 1,518,628 discloses that a viscosity stabilized, aqueous polysaccharide solution can be prepared if the aqueous liquid is first treated to remove dissolved oxygen, followed by addition of a water-soluble, sulfur-containing antioxidant, and a water-soluble readily oxidizable alcohol. The aqueous liquid is any fresh or saline water but is preferably a relatively soft water having a total dissolved salt content of not more than about 5,000 ppm and a hardness of not more than about 500 ppm. Lipton, SPE paper No. 5099, 49th annual meeting of SPE of AIME, Houston (1974) teaches that the injectability of the biopolymer derived from Xanthomonas campestris can be improved by proper mixing techniques, surface preservation and use of bacterial-free systems. Oxygen scavengers are added to prevent gel formation associated with the presence of Fe.sup.3+ ions. The oxygen scavenger prevents oxidation of Fe.sup.2+ to Fe.sup.3+. Patton, SPE paper No. 4670, 48th annual meeting of SPE of AIME, Las Vegas (1973) discloses that caustic treatment produces more filterable solutions. Sparged steam is recommended as a heating and deaerating agent to avoid local overheating and oxidative polymer decomposition. U.S. Pat. No. 3,800,877 describes the improved mobility control of aqueous solutions containing partially hydrolyzed polyacrylamides by incorporating water-soluble aldehydes into the aqueous solution. The aldehydes are stated to form complexes with oxygen thus overcoming the adverse effects of oxygen on the mobility control agent.
U.S. Pat. Nos. 3,625,888 and 3,764,548 both relate to oxygen scavenger compositions which are useful for reducing or inhibiting the corrosive effects of oxygen in water-flooding processes on oil field equipment. U.S. Pat. No. 3,976,593 teaches that bisulfites of corrosion inhibiting amines function as both corrosion inhibitor and oxygen scavenger. This system can be used in water-flooding processes for oil recovery.
Finally, U.S. Pat. No. 4,182,860 and 4,256,590 concern a method of producing a modified heteropolysaccharide which can be readily filtered and separated from other cell debris. The process involves the preparation of an aqueous, saline heteropolysaccharide solution containing at least 0.5 wt.% of at least one inorganic salt and heat treatment at temperatures of at least about 100.degree. C.
The process of U.S. Pat. No. 4,182,860 suffers from the disadvantage that prolonged heating at high temperatures results in a gradual loss of viscosity over a period of time. There is a need for a modified saline heteropolysaccharide which has stable viscosity properties at high temperatures for prolonged periods of time, particularly with respect to use in enhanced oil recovery from high temperature formations.